agents: remove the hive-level /effort control too

mara on #5034: "effort shares /models fate". The per-agent web UI no
longer sets effort: the `/effort` slash command, the effort picker,
`POST /api/effort`, `Bus::set_effort`, the `hyperhive-effort` writer,
`EFFORT_LEVELS`/`is_valid_effort` and the ACP effort-list payload are
gone, and `lib/modelEffort.ts` with them. The effort chip is
display-only and shows its source (`effort_source` in `/api/state`);
on ACP it stays hidden while the session offers no effort level.

The read side is unchanged: with no swarm value an agent runs an
existing `hyperhive-effort` file, else its agent.nix effort, else
medium. Existing files are not touched.

Refs #5034

nix/agent-modules/claude-settings.nix

@@ -274,7 +274,7 @@ in
     # un-overridable by user/project/CLI) — so the harness doesn't pass
     # `--settings` or copy the blob per turn. effortLevel is
     # deliberately NOT shipped here: effort is controlled live via the
-    # `--effort` CLI flag (HIVE_DEFAULT_EFFORT / the per-agent UI slider),
+    # `--effort` CLI flag (HIVE_DEFAULT_EFFORT / the swarm's effort setting),
     # which managed scope would otherwise override and lock.
     # Hive-enforced settings merged with a per-agent `env` block at BUILD
     # time via `jq` (not eval-time `readFile`, which would be import-from-